It would be likely that our solar system would not exist in other universes in my rerun experiment if our universe is stochastic. The early universe was not completely uniform. There were small variations in the intensity of energy, and in the density of the fundamental particles that formed. Some parts of the very young universe consequently had more quarks and electrons than others, and this variation translated into the distribution of galaxies across the night sky. Mathematical models suggest this early variation in the distribution of matter might have been caused by quantum randomness. In a stochastic universe, such randomness would result in a different layout of galaxies each time a universe like ours formed. There would be the same distribution of small and large galaxies, spiral and elliptical ones, but each galaxy in each universe would be different. Only our universe would have the Milky Way, the sun, the Earth and the other planets in our solar system.
Although unique, the Milky Way is not unusual. It is a galaxy of average size. Similarly, our solar system is one of a kind, but it too is not odd. Our sun is only a little larger than the average star, and observations of other stars through telescopes reveal that the planets that orbit our star are not particularly unusual. NASA reports that at least one in six stars has an Earth-sized planet, but some of these are orbiting stars that are very different from our sun. Another study has revealed that about one in five sun-sized stars have Earth-sized planets in their habitable zones. In contrast, only about one in twenty stars that are the size of our sun are orbited by gas giants like Jupiter and Saturn, planets that played such a key role in determining Earth’s current orbit. We can conclude that systems like ours, with planets like Earth that might be able to support life, are not particularly unusual in our galaxy, and we would expect them to occur at the same frequency in other galaxies in our universe.
Assuming four forces of the right strength and a large universe, Earth-like planets orbiting the right distance from sun-like stars for life to form appear to be quite likely. However, the suitability of Earth for life is dependent not just on its position in the habitable zone but also on its orbit and speed of spin. We currently do not have the data to study these attributes of other Earth-like planets, although scientists have proposed ways of doing these studies, so the data may be available in the not-too-distant future. But as I write we do not know whether our twenty-four-hour days are typical or unusual. Mars, our closest planet, has a day length very similar to that of Earth, but the next closest rocky planet, Venus, has a day length of 5,832 hours. Much more work is required, but the number of Earth-like planets suggests that some must have orbits and rates of spin appropriate for life.
The moon is also important for life on Earth, helping stabilize the climate by reducing our planet’s orbital wobble. We do not have the technological refinement to count moons around Earth-like planets in other star systems, but we do know they are common around the planets in our solar system, with over 200 moons counted. They can form at the same time as the planet, be captured by planets having started their lives as meteors and comets or even moons of other planets that escaped their orbits, or via a collision, as happened to produce our moon. We are the closest planet to the sun to have a moon, and our moon is the fifth largest in the solar system. Our moon may yet prove to be a little unusual, but it seems likely that some Earth-sized planets around other stars could well have one or more moons.
Part of the reason we exist is that the Earth has the right orbit, the right day length, the right axial tilt and a moon. Are these properties unusual? More observations of the heavens will help answer this question. Even if we conclude that close twins of Earth are rare, the cosmos is so large there may still be hundreds of thousands, or millions, of them in our galaxy alone. Yet life may not need an Earth-like planet to get going. Life could potentially arise on planets very different from Earth, or even on moons that harbour liquid water. Enceladus, a moon of Saturn, and Europa, a moon of Jupiter, both have salty oceans of liquid water below a thick layer of ice. Mars had surface oceans similar to those found on Earth today, although by 3 billion years ago they had been lost to space. Another four or five moons of the outer planets potentially have oceans trapped below a thick layer of ice. Could life have evolved on an ancient Mars or on these far-flung moons? We have no evidence that it did, but we have also been unable to look in any detail, so what can we say about life? Is the universe teeming with life, or is restricted to Earth?
The building blocks of life – nucleobases, amino acids and other organic compounds – readily form in space as revealed through analyses of meteors, meteorites and space dust. These molecules also form on Earth. The key molecular building blocks needed to get life started are consequently common, but that does not mean life routinely springs into existence – an energy input is needed. Volcanic energy sources likely powered the emergence of life on Earth. Volcanoes are not unique to our planet. Volcanoes, some dead, have been observed on Mercury, Venus, the Moon, Mars, and Jupiter’s moon, Io. Quite how volcanic energy and organic compounds combine to produce life is unclear, although scientists have plausible hypotheses of how life got started. However, they have not been able to identify the requisite conditions for it to get going in the lab. Nonetheless, membranes readily form, autocatalytic chemical reactions are not uncommon, and simple metabolism seems inevitable. What we don’t know is how easy it is for these key components of life to combine.
Although we do not know the conditions required for life to get started, there is no reason to suspect that the conditions on early Earth were particularly unusual. We have no reason to suspect that the emergence of life is a rare phenomenon. Of course, we cannot rule out that something extraordinarily unusual is required for life to get started and that only happened on Earth, but in the absence of any evidence of this, it seems reasonable to conclude that primitive life is a common feature of the universe. Whether it always uses RNA and DNA is unknown. Perhaps self-assembly manuals can be written in other molecular codes, but given amino acids are found on comets and meteors, my suspicion is that life will usually use RNA and DNA for coding instructions on how to build a cell. But much as French, English and Spanish use very similar alphabets to form a different language, the same base-pair triplets will not necessarily code for the same amino acids in independently evolved DNA-based life. Cytosine–adenine–guanine codes for glutamine on Earth, but it might code for valine or serine elsewhere. Or perhaps a completely different set of amino acids will be used.
Once life begins, competition between individuals, either of the same or of different species, is inevitable. The competition will always be for resources, and it leads to the evolution of life forms that can thrive on a wider range of resource types or exploit new resources. Evolution will always select for individuals that are more efficient at detecting, acquiring and utilizing these resources. Competition between life forms (where they exist) will be pervasive throughout the universe as the fittest replicator will always displace those that are less fit. On Earth, this competition has led to an increase in diversity and complexity in the eukaryote branch of the tree of life. As life diversifies, some of it may inevitably increase in complexity, as competition from a more diverse array of competitors will select for strategies to thrive in ever-more complex environments. Once life gets started, I suspect it will routinely proliferate and increase in complexity.